Differential recording of upper and lower cervical N13 responses and their contribution to scalp recorded responses in median nerve somatosensory evoked potentials.

To distinguish the different origins of cervical N13 potentials in median nerve somatosensory evoked potentials (SSEPs), cervical N13 potentials were recorded by two different montages. The abnormal patterns of the SSEPs were compared to the abnormal evoked spinal cord responses (ESCPs) recorded from posterior epidural space in 13 patients with various cervical lesions. SSEPs from the posterior cervical surface were recorded from the mid-cervical level with anterior neck reference (Cv5-AN) and from the upper cervical level with inion reference (Cv2-IN). Scalp responses were recorded from the parietal region contralateral to the stimulating side with non-cephalic reference (shoulder contralateral to stimulating side). ESCPs were recorded from the posterior epidural space using catheter electrodes or needle electrodes inserted into the ligamentum flavum. Lower cervical N13 (LC-N13) recorded from the Cv5-AN montage showed similar latency to upper cervical N13 (UC-N13) recorded from the Cv2-IN montage. The latency of the early part of the P13-P14 complex in the scalp montage was similar to that of the UC-N13 and the negative peak latency of the ESCPs recorded at the C2-3 level. Attenuation of the LC-N13 and relatively preserved UC-N13 and P13-P14 were characteristic in patients with cervical syringomyelia and compression cervical myelopathy at the mid-cervical levels. Attenuation of the UC-N13 with normal LC-N13 was characteristic in patients with cervical spondylotic myelopathy who showed conduction blockade of the ESCPs at the C3-4 level. In a patient with schwannoma at the C1-2 level, conduction blockade of the ESCPs was observed at the C1-2 level. P13 was normal but P14 was prolonged. UC-N13 and P13 latencies were similar to the negative peak latency of the ESCPs at the C2-3 level. We demonstrated that two different cervical N13 potentials can be recorded by two different montages and they represent different behavior in various spinal cord lesions. In addition, at least the early part of the P13-P14 complex originates in the upper cervical region. To distinguish two different cervical N13, it is useful to detect not only the cervical pathology but also the symptomatic cervical cord compression level in patients with cervical myelopathy.